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RESEARCH MEMORANDUM 



ELEVATED-TEMPERATURE COMBINED STRESS-RUPTURE PLUS 

FATIGUE STRENGTH OF WASPALOY HAVING DIFFERENT 

AGING TREATMENTS AND/OR MOLYBDENUM CONTENTS 

By C. A. Hoffman and M. B. Hornak 

Lewis Flight Propulsion Laboratoiy 
Cleveland, Ohio 

UNIVERSITY OF FLORIDA 
DOCUMENTS DEPARTMENT 
120 MARSTON SCIENCE LIBRARY 
P.O. BOX 117011 
GAINESVILLE, FL 32611-7011 USA 

NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 

WASHINGTON 

February 25, 1958 



f^/l 03 3- l'^ 3?^ ^^''^ 



NACA RM E57K22a 



NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 

RESEARCH MEMORANDUM 

ELEVATED-TEMPERATURE CCMBINED STRESS -RUPTURE PLUS FATIGUE 

STRENGTH OF WASPALOY HAVING DIFFERENT AGING 

TREATMENTS AOT)/OR MOLYBDENUM CONTENTS 

By C. A. Hoffman and M. B. Hornak 

SUMMARY 

An investigation was conducted to determine if the combined stress- 
rupture plus fatigue strengths of three groups of Waspaloy with different 
aging treatments and/or molybdenum contents could be correlated with 
their resultant stress -rupture ductilities and notch-rupture strengths. 

Fatigue tests were run at 1500° F with direct tensile cyclic stresses 
superimposed upon direct tensile mean stresses. Mean" tensile stresses 
were selected from 1500° F stress-rupture data to produce stress-rupture 
failure times of about 10, 100, and 500 hours. Cyclic tensile stress 
levels were chosen to equal 12.5 to 90 percent of the mean tensile 
stresses. 

A possible direct relation between combined stress -rupture plus 
fatigue strength and stress -rupture ductility of Waspaloy was indicated. 
However, no clear relation between combined stress-rupture plus fatigue 
strength and notch-rupture strength was found. If heat treatment and/or 
composition alone (i.e., without regard to their effects upon basic prop- 
erties such as ductility, etc.) are considered, the results indicate that 
the double-aging heat treatment (compared to a single age) increases the 
combined stress-rupture plus fatigue strength of Waspaloy and that the 
double-aging treatment plus increased molybdenum content causes further 
improvement in the combined stress -rupture plus fatigue strength of 
Waspaloy. 

INTRODUCTION 

Waspaloy is of interest as a gas turbine-bucket alloy because of its 
low strategic material content and its relatively good stress-rupture 
properties. The developer of this alloy had investigated the stress- 
rupture strength, stress -rupture ductility, and notch-rupture strength 



NACA R14 E57K22a 



of three lots of Waspaloy; these lots had different aging treatments 
and/or molybdenum contents. For 1500° F, the data revealed that the 
three lots possessed fairly similar stress-rupture strengths but dif- 
fered in both notch-rupture strength and stress -rupture ductility. Two 
of these lots, group A (single aged) and group B (double aged), which 
were from the same heat and contained 3 percent molybdenum, were notch 
strengthened; the third lot (group C) which was double aged and con- 
tained 7 percent molybdenum, either had the same strength or was slightly 
weaker when notched. The first lot of Waspaloy possessed the lowest 
stress-rupture ductility whereas the third lot had the highest stress- 
rupture ductility. Since the third lot had the best stress-rupture duc- 
tility but not the best notch-rupture strength, stress-rupture ductility 
and notch-rupture strength might not be directly related. 

One might hypothesize that among groups of material having comparable 
stress -rupture properties, the group exhibiting either or both the great- 
est stress-rupture ductility or notch-rupture strength would have the 
greatest time to failure in combined stress-rupture and fatigue (partic- 
ularly where the fatigue component is quite large); hence, a preliminary 
study of the foregoing three groups of Waspaloy was carried out to deter- 
mine if a correlation might exist between combined stress-rupture plus 
fatigue life and stress-rupture ductility and/or notch-rupture strength. 

In this investigation, endurance tests were run at 1500° F with 
direct tensile vibratory stresses superimposed upon direct tensile mean 
stresses for three lots of Waspaloy. The amplitude of the cyclic stresses 
was as great as 90 percent of the mean stresses. 



MATERIALS, APPARATUS, AND PROCEDURE 

Pratt & Whitney (who developed Waspaloy) furnished heat-treated 
stress -rupture specimens. These specimens were from the same lots and 
had the same heat treatments developed by Pratt <y Whitney for the speci- 
mens referred to in the INTRODUCTION. The chemical analyses and the de- 
scription of the heat treatments furnished with the specimens are given 
in tables I and II, respectively. Grain size and initial hardnesses ob- 
tained at the NACA Lewis laboratory are also presented in table II. For 
convenience, the designations A, B, and C will continue to be used for 
the single aged - 3 percent molybdenum, double aged - 3 percent molyb- 
denum, and double aged - 7 percent molybdenum groups, respectively (ta- 
bles I and II) . 

The specimens used in this investigation are illustrated in figure 
1. The surface finish of the specimen test section was 5 to 10 micro- 
inches root mean square. 



KACA RM E57K22a 



Commercial direct tensile stress fatigue machines operating at ap- 
proximately 2000 rpm (fig. 2) were used. A description of these machines 
together with the operating procedure is presented in references 1 and 2. 

Combined stress tests were run at the Lewis laboratory on specimens 
at 1500° F with direct tensile cyclic stresses superimposed upon direct 
tensile mean stresses. The mean tensile stresses were selected to yield 
1500° F stress-rupture lives of approximately 10, 100, and 500 hours at 
zero vibratory stress. The mean stresses selected to give these above 
approximate lives differed somewhat for each of the three groups of spe- 
cimens. The amplitudes of the cyclic tensile stresses were chosen to 
equal 12.5, 25, 50, 87, and 90 percent of the mean tensile stresses. The 
stress-rupture ductility, the notch-rupture, and a portion of the stress- 
rupture data used herein are those already referred to in the INTRODUCTION. 

A statistical survey of all tests showed that during any one test, 
the average values of the maximum and minimum loads (i.e., mean loads) 
were generally within 1 percent of the desired loads and the extreme 
values of the maximum and minimum loads were generally within 5 percent 
of the desired loads. (in general, the extreme values of the maximum or 
minimum unit stresses were within 1500 psi of the desired values with 
the greater number of the tests controlled within 1000 psi.) 

Samples of as -received Waspaloy were examined to determine the micro- 
structure of groups A, B, and C after heat treatment and prior to testing. 

Fractured specimens were examined macroscopically and classified 
according to the type of failure (stress-rupture, stress-rupture plus 
fatigue, and fatigue) as described in reference 2. Microspecimens were 
mounted to show typical intergranular or transgranular cracking and to 
show fracture surface appearance for each of the three groups of Waspaloy. 

Diameters of the failed specimens were determined at the failure 
zone. These data were used to compute the percent reduction in area at 
failure. Hardness measurements, obtained in Rockwell-A units and con- 
verted to Rockwell-C units, were taken on a transverse section 1/16 to 
l/s inch below the fracture surface. 



RESULTS AND DISCUSSION 

Combined Stress-Rupture Plus Fatigue Tests 

The metallurgical data presented in table II show that group C 
Waspaloy (7 percent molybdenum and double aged) had higher stress -rupture 
ductility as mentioned previously and somewhat higher hardness than groups 
A and B (3 percent molybdenum alloy, single aged and double aged, 
respectively) . 



NACA EM E57K22a 



Both 1500° F stress-rupture and notch-rupture data are plotted in 
figure 3. It can be noted from this figure that the NACA stress-rupture 
data tend to lie somewhat above the Pratt & Whitney data for groups A 
and C, whereas for group B the data lie on the same curve. The differ- 
ence in the case of groups A and C are likely random effects. Hence, a 
single rupture curve has been drawn for each group. A study of this 
figure indicates that groups A and B are notch strengthened in stress 
rupture, while group C is either unaffected or slightly notch weakened. 
The stress -rupture properties of the three groups can be considered es- 
sentially the same in view of the small observed differences. 

The results of this investigation are presented in figure 4 as a 
plot of the mean stress against time to failure at 1500 F with the 
cyclic stress ratio held constant. (The cyclic stress ratio is defined 
as the ratio of the amplitude of the cyclic stress to the mean stress.) 
In order to conveniently evaluate the data presented in figure 4, the 
data have been cross-plotted in figure 5 as cyclic stress ratio against 
time to failure at 1500° F and constant mean stress. A study of figure 
5 indicates that for the conditions studied, stress ratios of a given 
magnitude generally caused greater reductions in the life of group A 
(notch strengthened) than in group B (notch strengthened) and had least 
effect upon group C (slightly notch weakened or unaffected). At the low 
mean stress (20,000 psi) and high cyclic stress ratios (0.50 to 0.90) 
groups B and C appeared to have considerably better resistance to com- 
bined stress than group A. 

Consideration of the foregoing results suggests that a capacity for 
notch-rupture strengthening does not necessarily result in improved com- 
bined stress-rupture plus fatigue strength. In fact, a tendency towards 
notch weakening may not necessarily be expected to be harmful. 

As can be noted from table II, the stress-rupture ductility of group 
C was appreciably better than of groups A or B and B was slightly better 
than A. Further, as indicated previously, group C had the best combined 
stress-rupture plus fatigue strength, group B had intermediate strength, 
and group A had the lowest strength. Thus, there is indication of a pos- 
sible relation between stress-rupture ductility and combined stress- 
rupture plus fatigue strength. 

Reductions in area at failure (dynamic ductility) in combined stress- 
rupture plus fatigue tests plotted against time to failure are presented 
in figure 6. A study of this figure indicates that group C is generally 
most ductile followed in ductility by groups B and A over the range of 
cyclic stress ratios and mean stress levels studied. Further, it can be 
noted that the ductilities of the three groups decreased as cyclic 
stresses increased and tended to become quite small (l to 2 percent) and 
about the same order of magnitude at the higher cyclic stress ratios. 
This particular behavior is quite interesting since it demonstrates that 



NACA RM E57K22a 



while differences in dynamic ductility (as measured herein) become very 
small, combined stress-rupture plus fatigue strengths still may differ 
considerably for the three groups. Hence, one might conclude that com- 
bined stress-rupture plus fatigue strength is not directly related to 
dynamic ductility. 

If the heat treatment and/or composition of the alloys studied in 
this investigation are considered alone (i.e., without regard to their 
resultant effects upon the basic properties of ductility, etc.), the fol- 
lowing observations may be made: Double aging has improved the combined 
stress-rupture plus fatigue strength of the Waspaloy with the 3 percent 
molybdenum content. Additional molybdenum in conjunction with the 
double-age heat treatment further improved the combined stress-rupture 
plus fatigue properties. 



Metallurgical Evaluation of Failed Specimens 

The structures of the three groups of Waspaloy after heat treatment 
and prior to testing are presented in figure 7. Groups A and B have 
about the same grain size, while group C average grain size is slightly 
finer. Group C has a duplex grain structure and pronounced spheroidiza- 
tion of the grain boundaries. Inasmuch as a nonuniform grain size struc- 
ture has generally been considered to cause reduced life in turbine 
buckets, the possibility of a duplex grain structure causing reduced 
life under conditions of combined stress-rupture plus fatigue may be 
raised. However, group C with a duplex grain structure is indicated as 
having as good or better strength than the groups with a uniform grain 
structure. 

The macroscopic appearance of the specimens at failure is summarized 
in table III. Photomicrographs illustrating the three types of failure 
are presented in figure 8. There did not appear to be any significant 
difference in the macroscopic failure behavior associated with the three 
groups of Waspaloy. 

Stress-rupture fracture appears to have been initiated intergranu- 
larly and then progressed in a predominately transgranular fashion. This 
was the case for all three groups of Waspaloy. Intergranular initiation 
of fracture is illustrated in figure 9(a) for groups A and B; figures 
9(b) and (c) illustrate intergranular initiation of cracking in group C, 
where the surface grains were fine and coarse, respectively. The progres- 
sion of fracture through groups A and B and through group C is illustrated 
in figures 10(a) and (b), respectively. 

The fatigue areas fractured transgranular ly in all three groups of 
Waspaloy. Fatigue fracture of groups A and B is illustrated in figure 
ll(a) and of group C in figure ll(b). Intergranular failure along the 
sides of fatigue-failed specimens was quite prevalent. 



MCA RM E57K22a 



Stress-rupture plus fatigue failures combined the metallographic 
features of both stress-rupture failed specimens and fatigue failed 
specimens. 

The microstructures of the failed specimens do not offer any appar- 
ent explanation for the differences in the combined stress strengths of 
the three groups of Waspaloy. 

The hardnesses at failure are presented in figure 12. These data 
do not show any pronounced trend between the hardness at failure and 
cyclic stress ratio; however, group C hardness values are higher than 
groups A and B. 



SUMMARY OF RESULTS 

This investigation, conducted 'to determine if the combined stress - 
rupture plus fatigue strengths of three groups of Waspaloy could be corre- 
lated with either their stress -rupture ductilities or notch-rupture 
strengths, yielded the following results: 

1. No clear relation occurred between combined stress -rupture plus 
fatigue strength and notch-rupture strength. Contrary to what might be 
expected, a group of specimens with least notch-rupture strength proper- 
ties (slightly notch weakened or unaffected) had the best combined 
stress-rupture plus fatigue properties. 

2. A possible relation between combined stress-rupture plus fatigue 
strength and stress -rupture ductility was observed. However, no relation 
between combined stress-rupture plus fatigue strength and dynamic ducti- 
lity (ductility at failure in combined stress) was observed. 

3. If the heat treatments and/or compositions of the alloys studied 
in this investigation are considered alone (i.e., without regard to their 
resultant effects upon the basic properties of ductility, etc.), the 
following may be observed: Double aging has improved the combined stress- 
rupture plus fatigue strength of the Waspaloy with the 3 percent molyb- 
denum content. Additional molybdenum, in conjunction with the double- 
aging treatment further improved the combined stress -rupture plus fatigue 
properties. 

4. The high molybdenum (7 percent) Waspaloy group had a duplex grain 
structure; this type of structure might be questioned as having a dele- 
terious effect upon resistance to combined stress -rupture plus fatigue. 
However, this group generally exhibited best combined stress strength. 



Lewis Flight Propulsion Laboratory 

National Advisory Committee for Aeronautics 
Cleveland, Ohio, December 10, 1957 



NACA RM E57K22a 



REFERENCES 

1. Ferguson, Robert F.: Effect of Magnitude of Vibratory Load Super- 

imposed on Mean Tensile Load on Mechanism of and Time to Fracture 

of Specimens and Correlation to Engine Blades. NACA RM E52I17, 1952. 

2. Hoffman, Charles A.: Strengths and Failure Characteristics of AMS 

5765A (S-816) Alloy in Direct Tensile Fatigue at Elevated Tempera- 
tures. Proc. ASTM, vol. 56, 1956, pp. 1063-1080. 



NACA EM E57K22a 



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NACA RM E57K22a 



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6 8 10 20 40 60 100 200 
Time to failure, hr 



400 600 1000 



(c) Groiip C (7 percent molybdenum, double aged). 
Figure 3. - Variation of time to failure at 1500° F with static stress. 



NACA EM E57K22a 



13 





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1000 



Time to failure, hr 

(c) Group C (7 percent molybdenum, double aged). 

Figure 4. - Variation of time to failure with cyclic mean stress at constant stress ratio and 1500° F. 



14 



NACA EM E5TK22a 




of^Ea ssaa^g oflojCo 



NACA EM E57K22a 



15 









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16 



NACA EM E57K22a 





/ 




^ 


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(a) Group A; transverse section; X750 



("b) Group Bj transverse section; X750 



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(c) Group Cj longitudinal section; XlOO 



(d) Group C; longitudinal section; X250 



Figure 7- - Structure after heat treatment and prior to testing; electrolytically 
etched with HC]>HN0_+H20. Magnification reduced 42 percent in reproduction. 



NACA RM E57K22a 



17 




(a) Stress-rupture failure ; X3 (b) Stress-ruptxire plus fatigue failure; X3.5 




C-46623 



(c) Fatigue failure; X3.5 



Figure 8. - Three types of failure of Waspalloy. Magnification 
reduced I7 percent in reproduction. 



18 



NACA BM E57K22a 






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3 



NACA EM E57K22a 



19 




(a) Fractiire in specimens of groups A and B. 




cMmfm^' 



m-^_ 







C-46625 



(t)) Fractiu-e in specimens of group C. 



Figure 10. - Stress-rupture fractiu-e surfaces. Electrolytically 
etched with HCL+HHOj+HgO ; X250. Magnification reduced 6 percent 
in reproduction. 



Load 



Load 



\ 



20 



MCA EM E57K22a 




i 





Load 



(a) Specimens of groups A and B. 



^^^^^^H 




^^^^HFractvire ^^H 


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C-46622 



(b) SpecimenB of group C. 

Figure 11, - Fatigue fractvire surfaces. Electrolytlcally etched with 
HCL+HNOg+H-O ; X250 . Magnification reduced 6 percent in reproduction. 



Load 



NACA EM E57K22a 



21 






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NACA - Langley Field, V. 



UNIVERSITY OF FLORIDA 



3 1262 08106 590 5 



^^«|ITYOFaOR/DA 
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